This invention relates to data display arrangements of a type for displaying data represented by digital codes, the displayed data being composed of discrete characters the shapes of which are defined by selected dots of a dot matrix which constitutes a character format for the characters.
Data display arrangements of the above type have application in the video terminals of a variety of different data display systems for displaying data on the screen of a CRT (cathode ray tube) or other raster scan display device. One such data display system, for instance, is used in conjunction with telephone data services which offer a telephone subscriber having a suitable video display terminal the facility of access over the public telephone network to data sources from which data can be selected and transmitted in digitally coded form to the subscriber's premises for display. Examples of this usage are the British and German videotex services Prestel and Bildschirmtext.
A data display arrangement of the above type includes, in addition to the CRT or other display device, acquisition means for acquiring transmission information representing data selected for display, memory means for storing derived digital codes, and character generator means for producing from the stored digital codes character generating signals for driving the display device to produce the data display.
It is known for the character generator means to include a character memory in which is stored character information identifying the available character shapes which the arrangement can display. This character information is selectively addressed in accordance with the stored digital codes and the information read-out is used to produce the character generating signals for the data display. Where, as would usually be the case, the display is on the screen of a CRT, this selective addressing is effected synchronously with the scanning action of the CRT.
To facilitate this selective addressing, it is convenient to store the character information that identifies the patterns of discrete dots which define the character shapes as corresponding patterns of data bits in respective character memory cell matrices. With this form of storage, the dot pattern of a character shape as displayed in a display frame on the screen of the CRT can have a one-to-one correspondence with the stored bit pattern for the character. The display frame may be produced with or without interlaced field scanning.
In order to facilitate further the aforesaid selective addressing, it is also convenient to display characters of a standard size arranged in character rows which can contain up to a fixed maximum possible number of characters. This standardisation determines the size for a rectangular character display area, composed of a plurality of dot rows, which is required for displaying one character. In general, the dot rows are displayed once in successive scanning lines in each field.
With a view to extending the display facilties of a data display arrangement of the above type, it has been proposed to provide a choice of different colours for displayed characters. For this proposal, additional stored data can be used to encode different colour choices.
Another proposal for extending the display facilities of the data display arrangement is to provide for the selective display of characters of double height. For this second proposal, a double height character will occupy two corresponding character display areas in adjacent character rows, that is, the display area for a double height character is doubled. However, in order to avoid having to store double height bit patterns in respect of double height characters, it is usual instead to modify the addressing of the existing stored bit patterns for normal height characters. This modified addressing is such as to cause each bit row of a character bit pattern to be read-out twice, so that the resultant dot row is displayed twice in successive scanning lines (in each field).
In order for a displayed row of characters to have an effective baseline which gives visual alignment to the row and below which the "tails" of descender letters, or base accents such as a cedilla can lie, it is known for a character display area to have a number of its dot rows at the bottom of the area not occupied by any part of a displayed character except for such a tail or accent. The intersection between these unoccupied dot rows and the remainder of the area where the main body of a character is displayed defines the baseline. A viewer is not normally aware of the positioning of the displayed characters within their respective display areas. Rather the eye is drawn to the baseline as thus defined, with descender tails and base accents apparently being located below the baseline. However, when a displayed character is made double height by using the modified addressing referred to above, the baseline for the displayed double height character becomes shifted with respect to the baseline for ordinary height characters due to the linear expansion (doubling) of the character height. Consequently, when a displayed character row comprises a combination of double height and normal characters, a problem occurs in that the visual baseline effect for the character row is destroyed.
Prior U.S. Pat. No. 4,321,596 discloses a method of aligning characters on the screen of a television receiver using an algorithm in which one step provides that when a row of characters contains both single (normal) height characters and double height characters, with none of the double height characters being descender letters, then the alignment of both types of characters in the row is the same as the alignment in a row containing only single height characters. However, another step of the algorithm provides that when a row of characters contains only double height characters then the alignment is offset upwards by two scanning lines with respect to the alignment of a row of normal height characters; and a further step of the algorithm provides that if a double height descender letter is in a character row containing both normal and double height characters the alignment of the double height characters is offset upwards by one scanning line with respect to the alignment of the normal height characters in the same row. In the last step, the last dot row of the double height descender letter is not repeated.
Therefore, although the problem of displaying double height descender letters is mitigated with this prior art method by the non-repetition of the "tails" thereof, there is nevertheless an interruption of the visual baseline effect because four different alignment criteria are used. It is an object of the present invention to provide a simpler means of overcoming this problem without destroying the visual baseline effect.